1. Field of the Invention
The present invention relates to power supply circuits including transistors as switches and semiconductor devices including the power supply circuits.
2. Description of the Related Art
A power supply circuit (also referred to as a power conversion circuit) has a function of regulating voltage supplied from a battery or a power supply such as a commercial power supply in accordance with the specification of a load. For example, voltage supplied from a commercial power supply is AC voltage having an effective value of 100 Vrms. In contrast, voltage needed in a commonly-used electronic device is a comparatively low DC voltage of 5 V, 12 V, or the like in many cases. In that case, voltage output from a power supply is rectified and stepped down by a power supply circuit and then supplied to a load.
In general, a power supply circuit includes a circuit regulating voltage, such as a rectifier circuit, a smoothing circuit, or a converter, and a switch that is formed using a semiconductor element such as a transistor and has a function of controlling electrical connection between a power supply and a load. Unlike a mechanical switch, a semiconductor switch formed using a semiconductor element has advantages of high reliability, long lifetime, and high-speed switching. Note that in the case where the power supply circuit includes the semiconductor switch, it is necessary to provide a control circuit for controlling the operation of the transistor in the power supply circuit, and voltage regulated in the circuit is supplied not only to the load but also to the control circuit.
Patent Document 1 discloses a stabilized power supply in which a battery is connected to an electronic circuit that is a load through an electronic switch to which a system control circuit is connected.